I
keep it on a tender in the garage, but the car does sit in the garage a lot. I know that sitting is not great for batteries, so maybe the recent cold pushed over the edge.
Sitting is not bad for lead acid storage batteries. Mine has sat for about 5 months per year disconnected / no tender without issue since 2012. Sitting with a load connected which drains the battery and causes it to go through a deeper charge cycle is bad. If the tender is large enough to supply the parasitic loads (all 35 ma of it) and correctly regulated the battery should float and be indifferent to the parasitic loads.
You didn't specifically say it; but, I presume that your battery drain issues led to a non start situation? Did that occur with the battery on the tender? If so, that suggest a problem with the tender.
In addition to the parasitic loads of the integrated control unit, clock stereo and the security unit, the ECU has keep alive power for fuel trims and error codes and the ABS, EPS, TCS all probably have keep alive power for there error codes. The earlier suggestion about disconnecting things to eliminate the parasitic loads is probably not practical. The parasitic current you measured seems reasonable so it does not seem like you have a short anywhere.
I 'think' the battery in my car is the one intended for the automatic (I forget the group number). The reason I say think is that the battery in the car expired about a year after I bought the car, so I took the battery into a battery shop to be recycled. They asked me for the application and brought out a battery significantly smaller than what I was recycling. I said ' No just give me one that's the same as what I brought in'. Hence my uncertainty about the actual group number on my battery. Short story, I have let the car sit for three + weeks in temperatures down to 0C without having starting issues and without using a tender. It has been like that since 2012. I also have a keyless entry system which increases the parasitic load. When the car is in the garage I don't lock it. I do use Mobil 1 which has a lower pour point than conventional oils which may ease the starting burden on the battery.
If you want to extend the cars sitting time, consider switching to the slightly larger battery used in the automatic version of the NSX. There may also be slightly larger batteries that will fit in the holder - there are sizing guides for the various group numbers on the web and you could work through the list to see what fits. The battery I purchased from the battery shop was their house brand, nothing special and not expensive. You could also try Brian K's idea and seek out a battery designed for deep cycle operation.
